I took a gander at Windows 7, as I'm sure people will be fawning and chastising it constantly in months and years to come. When you look at Windows vs. OS X, the interfaces are obviously different, and OS X has traditionally improved the computer user's experience. It's simple to use. Why is that? Because OS X focused on the end user first.
Windows, however, is a standard. Everyone uses it (80% can be everyone in this example), therefore everyone is familiar with it. I'm not saying Windows is perfect. And I doubt an OS X user will say OS X is perfect. If they do (on either "side") they are total fanboys and you should slap their Starbucks drink right from their hands.
But I don't really want to delve into a Win vs. OS X argument. What I want to get into is the fundamental flaws in approaching an OS design. If you want to make it simple, "simple" is not a code optimization issue. Because "simple" is defined from the user, not the developer. Case in point, the sample screen shot of Windows 7 (below) have a base-level usability error. The screen capture below shows what the user reads (in red) and the user action (in yellow). The blue arrow is where the action goes.
Even though Windows 7 is in Beta, layout issues such as this are why OS X users feel more comfortable. User design is first, then functionality. Windows has such a large user base (therefore slow to change), it's likely simple design changes would be met with backlash.